Electrochemical reduction of metal oxides

ABSTRACT

A process for electrochemically reducing metal oxide feed material in a solid state is disclosed. The process includes the steps of agitating an electrolyte and metal oxide powders in the electrolyte and applying an electrical potential across a cathode in contact with the electrolyte and an anode and electrochemically reducing the metal oxides.

The present application is a continuation-in-part application of and claims priority to PCT/AU2005/000409 published in English on Sep. 29, 2005 as PCT WO 2005/090640 and from AU 2004901524 filed Mar. 22, 2004, the entire contents of each are incorporated herein by reference in their entirety.

The present invention relates to electrochemical reduction of metal oxides. The present invention relates particularly to electrochemical reduction of metal oxides in the form of powder to produce metal having a low oxygen concentration, typically no more than 0.2% by weight.

BACKGROUND

The present invention was made during the course of an on-going research project on electrochemical reduction of metal oxides being carried out by the applicant. The research project has focused on the reduction of titanium oxide, more specifically titania (TiO₂).

During the course of the research project the applicant has carried out a series of experiments investigating the reduction of titania in electrolytic cells comprising a pool of molten CaCl₂-based electrolyte, an anode formed from graphite, and a range of cathodes.

The CaCl₂-based electrolyte used in the experiments was a commercially available source of CaCl₂, namely calcium chloride dihydrate, which decomposed on heating and produced a very small amount of CaO.

The applicant operated the electrolytic cells at a potential above the decomposition potential of CaO and below the decomposition potential of CaCl₂.

The applicant found that the cells electrochemically reduced titania to titanium with low concentrations of oxygen, i.e. concentrations less than 0.2 wt. %, at these potentials.

The applicant operated the cells under a wide range of different operating set-ups and conditions.

The present invention was made unexpectedly in two experiments on sub-micron powders of pigment grade titania. The powders were mixed with molten CaCl₂-based electrolyte containing CaO in electrolytic cells comprising anodes and cathodes in contact with the electrolyte/powder baths. The applicant found unexpectedly that the titania powders were successfully reduced in the molten electrolyte baths. The applicant also found unexpectedly that there was very little carbon produced in the experiments that was retained in the cells—this is a potentially important finding given that carbon contamination can be significant. The applicant had not expected to achieve these results.

The unexpected success of the experiments opens up the possibility of commercial production of metals from metal oxides such as titania on a far more straightforward basis than was hitherto thought to be possible.

BRIEF SUMMARY

According to the present invention there is provided a process for electrochemically reducing metal oxide feed material in a solid state, which includes agitating an electrolyte and metal oxide powders in the electrolyte and applying an electrical potential across (a) a cathode in contact with the electrolyte and (b) an anode and electrochemically reducing the metal oxides.

Additional features and benefits of the present invention are described and will be apparent from the accompanying drawings and detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate embodiments of the invention according to the practical application of the principles described and shown.

FIG. 1 is a schematic of an apparatus for practicing the present invention.

DESCRIPTION

The present invention includes a process for electrochemically reducing metal oxide feed material in a solid state. The process includes agitating an electrolyte and metal oxide powders in the electrolyte and applying an electrical potential across (a) a cathode in contact with the electrolyte and (b) an anode and electrochemically reducing the metal oxides. The applicant speculates that the agitation caused intermittent contact between the powder particles and the cathode that was sufficient to enable reduction of titania powders and restrict sintering of the powder particles together that could adversely affect the reduction of unreduced or partially reduced powders.

Preferably, the particle size of the powders is selected so that the electrolyte and the powders form a slurry, i.e. a two phase mixture, in which the powder particles are suspended in the electrolyte.

The electrolyte and the metal oxide powders may be agitated by any suitable means. By way of example, the electrolyte and the metal oxide powders may be agitated by physical means, such as a stirrer. Alternatively, or in addition, the electrolyte and the metal oxides powders may be agitated by gas injection.

The applicant has found in the two experiments described above that gas injection enabled segregation of carbon contaminant formed in the experiments to the surface of the bath and titanium to the bottom of the bath. This is an important feature in terms of separating carbon and titanium in the process.

The metal oxide powders may be any suitable metal oxide. As is indicated above, the present invention has particular application to solid state reduction of titanium oxide particles, especially titania particles.

Preferably, the electrolyte is a CaCl₂-based electrolyte containing CaO. In the case of a CaCl₂-based electrolyte containing CaO, preferably the powders are sub-micron size. In the case of a CaCl₂-based electrolyte containing CaO, preferably the process comprises applying a potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCl₂.

The process may be carried out on a batch basis, a semi-continuous basis, and a continuous basis. The process may be carried out by positioning a member such as bar, plate, or sheet in contact with the electrolyte so that reduced powders can deposit on the member. With this arrangement, the process comprises removing the member from the electrolyte and stripping deposited reduced powders from the members. The process may be carried out in a cell that contains a bath of electrolyte and metal oxide powders, an anode, and a cathode.

The anode may be made from any suitable material. The anode may be a consumable or a non-consumable anode. Typically, the anode is a consumable anode. The cathode may be made from any suitable material.

The process may be carried out as a multi-stage process with electrolyte and partially reduced and unreduced powders in a slurry form being transferred from a first stage to one or more than one successive stage in the process and being reduced in each stage. The multi-stage process may be carried out in the above-described cell, with discharge and recycling of the slurry to the cell. The multi-stage process may be carried out in a series of the above-described cells.

The process is not confined to being carried out in the above-described cell. By way of example, the process may be carried out on a continuous basis by passing a slurry of the electrolyte and metal oxide powders through a reactor, such as a pipe reactor, that defines a pathway for flow of the slurry between an inlet and an outlet and includes one or more than one anode and one or more than one cathode along the length of the pathway. The reactor may include agitating the slurry by a means, such as baffles or the like in the pathway that causes the slurry to flow in a turbulent flow pattern along the pathway. Alternatively, or in addition, the process may include agitating the slurry by introducing the slurry in a turbulent flow into the reactor.

When operated on a continuous basis preferably the process comprises separating reduced powders from the electrolyte downstream of the outlet of the pathway and processing the reduced powders as required.

According to the present invention there is also provided an apparatus for electrochemically reducing metal oxide powders, such as titanium oxide powders, which includes (a) a means for containing a bath of a molten electrolyte and metal oxide particles in the electrolyte, (b) a cathode in contact with the electrolyte, (c) an anode, (d) a means for applying a potential across the anode and the cathode, and (e) a means for agitating the electrolyte. The apparatus may be adapted to operate on a batch basis, a semi-continuous basis, or a continuous basis.

Preferably, the means for applying a potential across the anodes and the cathode includes (a) a power source and (b) an electrical circuit that electrically interconnects the power source, the anodes, and the cathode.

The basic cell configuration of (a) a means for containing a bath of a molten electrolyte and metal oxide particles in the electrolyte, (b) a cathode in contact with the electrolyte, (c) an anode, (d) a means for applying a potential across the anode and the cathode is as described by way of example in other patent families of the applicant such as US 2004/0247478 (WO2003/016594), WO2003/076690, US 2006/0180462 (WO2004/035873) and US 2005/0050989 (WO2004/053201), each of which is incorporated herein by reference.

Turning to FIG. 1, a batch apparatus is shown. The apparatus 10 includes a crucible 12 for containing a bath of molten electrolyte 14 and metal oxide particles 16 in the electrolyte 14. The crucible may have a lid 13. The crucible 12 and lid 13 may be formed from any suitable material such as graphite. A cathode 18 is in contact with the electrolyte 14 and may optionally have an insulator 20 surrounding the cathode. An anode is formed from the crucible by a rod 22, which may be formed from stainless steel and may be in electrical contact with a DC power supply 24 and the crucible 12. A thermocouple 26 may be provided in close proximity to the cathode 18. The thermocouple may also be surrounded by an insulator 20.

As is indicated above, the present invention was made unexpectedly in two experiments in which sub-micron powders of pigment grade titania were reduced in electrolytic cells containing baths of molten CaCl₂-based electrolyte containing CaO, anodes and cathodes.

In the first experiment, the anode and the cathode were arranged to extend into the cell and the cathode had a relatively large surface area compared to the size of the cell. The titania powders were 10% by weight of the total weight of the powders and the electrolyte. The titanium powders were sub-micron sized. The cell was operated at a constant potential of 3 V for a period of 7 hours. The cell achieved currents of up to 8 A during the experiment.

In the second experiment, which was run after the success of the first experiment, the wall of the cell formed the cathode and the anode was arranged to extend into the cell. The solids loading was 5% in this experiment. In other words, the titania powders were 5% by weight of the total weight of the powders and the electrolyte. The titania powders were sub-micron size. The cell was operated at a constant potential of 3 V for a period of 7 hours. The cell achieved currents of up to 30 A during the experiment.

In both experiments: (i) the bath was agitated during the course of the experiments by inert gas injection to ensure movement of the powders within the bath; (ii) the operating potential of 3 V is a potential above the decomposition potential of CaO in the electrolyte and below the decomposition potential of CaCl₂; and (iii) there was a build-up of reduced powders on the cathode, although without any apparent sintering of reduced powders that restricted reaction rates or otherwise adversely affected the experiments.

Some regions of the titania powders were reduced by up to 50% by the end of the first experiment. The titania powders were reduced up to 95% by the end of the second experiment. At the end of the second experiment, the applicant allowed the cell to cool to room temperature and then sectioned the cell. The applicant found that the cell comprised a layer of titanium metal powders on the bottom wall of the cell and a layer of substantially “clean” electrolyte on the metal layer. The applicant also found that the side walls of the cell had a layer of titanium carbide.

Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention. While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. 

1. A process for electrochemically reducing metal oxide feed material in a solid state comprising agitating an electrolyte and metal oxide powders in the electrolyte and applying an electrical potential across (a) a cathode in contact with the electrolyte and (b) an anode and electrochemically reducing the metal oxides.
 2. The process defined in claim 1 includes agitating an electrolyte and metal oxide powders to an extent required to cause intermittent contact between the powder particles and the cathode that is sufficient to enable reduction of titania powders and restrict sintering of the powder particles together that could adversely affect the reduction of unreduced or partially reduced powders.
 3. The process defined in claim 1 includes selecting the particle size of the powders so that the electrolyte and the powders form a slurry in which the powder particles are suspended in the electrolyte.
 4. The process defined in claim 1 includes agitating the electrolyte and the metal oxide powders by physical means or by gas injection.
 5. The process defined in claim 1 wherein the powders are sub-micron size.
 6. The process defined in claim 1 wherein the electrolyte is a CaCl₂-based electrolyte containing CaO.
 7. The process defined in claim 6 includes applying a potential across the anode and the cathode that is above the decomposition potential of CaO and below the decomposition of CaCl₂.
 8. The process defined in claim 1 includes positioning a member in contact with the electrolyte so that reduced powders can deposit on the member.
 9. The process defined in claim 8 includes removing the member from the electrolyte and stripping deposited reduced powders from the member.
 10. The process defined in claim 1 wherein the process is a multi-stage process with electrolyte and partially reduced and unreduced powders in a slurry form being transferred from a first stage to one or more than one successive stage in the process and being reduced in each stage.
 11. The process defined in claim 1 wherein the process is a continuous process that includes passing a slurry of the electrolyte and metal oxide powders through a reactor that defines a pathway for flow of the slurry between an inlet and an outlet and includes one or more than one anode and one or more than one cathode in the pathway.
 12. The process defined in claim 11 includes agitating the slurry to cause the slurry to flow in a turbulent flow pattern along the pathway.
 13. The process defined in claim 11 includes agitating the slurry by introducing the slurry in a turbulent flow into the reactor.
 14. The process defined in claim 11, when operated on a continuous basis, includes separating reduced powders from the electrolyte downstream of the outlet of the pathway and processing the reduced powders as required.
 15. The process defined in claim 1 wherein the metal oxide particles are titanium oxide particles.
 16. An apparatus for electrochemically reducing metal oxide powders comprising: (a) a means for containing a bath of a molten electrolyte and metal oxide particles in the electrolyte, (b) a cathode in contact with the electrolyte, (c) an anode, (d) a means for applying a potential across the anode and the cathode, and (e) a means for agitating the electrolyte.
 17. The apparatus defined in claim 16 wherein the means for applying the potential across the anodes and the cathode includes (a) a power source and (b) an electrical circuit that electrically interconnects the power source, the anodes, and the cathode. 